diff --git a/docs/langref.rst b/docs/langref.rst
index 61c534ad69..0f49b8f6ed 100644
--- a/docs/langref.rst
+++ b/docs/langref.rst
@@ -817,14 +817,7 @@ represented as a floating point number.
 Conversion operations
 ---------------------
 
-.. inst:: a = bitcast x
-
-    Reinterpret the bits in ``x`` as a different type.
-
-    The input and output types must be storable to memory and of the same size.
-    A bitcast is equivalent to storing one type and loading the other type from
-    the same address.
-
+.. autoinst:: bitcast
 .. inst:: a = itrunc x
 .. inst:: a = uext x
 .. inst:: a = sext x
diff --git a/meta/cretonne/base.py b/meta/cretonne/base.py
index fd3e4fdcb5..15f23b1abc 100644
--- a/meta/cretonne/base.py
+++ b/meta/cretonne/base.py
@@ -761,4 +761,29 @@ nearest = Instruction(
         """,
         ins=x, outs=a)
 
+
+#
+# Conversions
+#
+
+Mem = TypeVar(
+        'Mem', 'Any type that can be stored in memory',
+        ints=True, floats=True, simd=True)
+MemTo = TypeVar(
+        'MemTo', 'Any type that can be stored in memory',
+        ints=True, floats=True, simd=True)
+
+x = Operand('x', Mem)
+a = Operand('a', MemTo, 'Bits of `x` reinterpreted')
+
+bitcast = Instruction(
+        'bitcast', r"""
+        Reinterpret the bits in `x` as a different type.
+
+        The input and output types must be storable to memory and of the same
+        size. A bitcast is equivalent to storing one type and loading the other
+        type from the same address.
+        """,
+        ins=x, outs=a)
+
 instructions.close()
diff --git a/src/libcretonne/instructions.rs b/src/libcretonne/instructions.rs
index 21138e2ce7..f6ca187511 100644
--- a/src/libcretonne/instructions.rs
+++ b/src/libcretonne/instructions.rs
@@ -432,6 +432,30 @@ impl ValueTypeSet {
         };
         allowed && self.is_base_type(typ.lane_type())
     }
+
+    /// Get an example member of this type set.
+    ///
+    /// This is used for error messages to avoid suggesting invalid types.
+    pub fn example(&self) -> Type {
+        if self.base != types::VOID {
+            return self.base;
+        }
+        let t = if self.all_ints {
+            types::I32
+        } else if self.all_floats {
+            types::F32
+        } else if self.allow_scalars {
+            types::B1
+        } else {
+            types::B32
+        };
+
+        if self.allow_scalars {
+            t
+        } else {
+            t.by(4).unwrap()
+        }
+    }
 }
 
 /// Operand constraints. This describes the value type constraints on a single `Value` operand.
@@ -506,4 +530,49 @@ mod tests {
         // that?
         assert_eq!(mem::size_of::<InstructionData>(), 16);
     }
+
+    #[test]
+    fn value_set() {
+        use types::*;
+
+        let vts = ValueTypeSet {
+            allow_scalars: true,
+            allow_simd: true,
+            base: VOID,
+            all_ints: true,
+            all_floats: false,
+            all_bools: true,
+        };
+        assert_eq!(vts.example().to_string(), "i32");
+
+        let vts = ValueTypeSet {
+            allow_scalars: true,
+            allow_simd: true,
+            base: VOID,
+            all_ints: false,
+            all_floats: true,
+            all_bools: true,
+        };
+        assert_eq!(vts.example().to_string(), "f32");
+
+        let vts = ValueTypeSet {
+            allow_scalars: false,
+            allow_simd: true,
+            base: VOID,
+            all_ints: false,
+            all_floats: true,
+            all_bools: true,
+        };
+        assert_eq!(vts.example().to_string(), "f32x4");
+
+        let vts = ValueTypeSet {
+            allow_scalars: false,
+            allow_simd: true,
+            base: VOID,
+            all_ints: false,
+            all_floats: false,
+            all_bools: true,
+        };
+        assert_eq!(vts.example().to_string(), "b32x4");
+    }
 }
diff --git a/src/libreader/parser.rs b/src/libreader/parser.rs
index 7977ccbf54..cf0efb1ffd 100644
--- a/src/libreader/parser.rs
+++ b/src/libreader/parser.rs
@@ -651,7 +651,10 @@ impl<'a> Parser<'a> {
                 } else if constraints.is_polymorphic() {
                     // This opcode does not support type inference, so the explicit type variable
                     // is required.
-                    return err!(self.loc, "type variable required for polymorphic opcode");
+                    return err!(self.loc,
+                                "type variable required for polymorphic opcode, e.g. '{}.{}'",
+                                opcode,
+                                constraints.ctrl_typeset().unwrap().example());
                 } else {
                     // This is a non-polymorphic opcode. No typevar needed.
                     VOID
diff --git a/tests/parser/tiny.cton b/tests/parser/tiny.cton
index e404985bf8..ae3466b618 100644
--- a/tests/parser/tiny.cton
+++ b/tests/parser/tiny.cton
@@ -42,3 +42,11 @@ ebb0(vx0: f32, vx1: f32):
     v1 = fcmp uno, vx0, vx1
     v2 = fcmp lt, vx0, vx1
 }
+
+; The bitcast instruction has two type variables: The controlling type variable
+; controls the outout type, and the input type is a free variable.
+function bitcast(i32, f32) {
+ebb0(vx0: i32, vx1: f32):
+    v0 = bitcast.i8x4 vx0
+    v1 = bitcast.i32 vx1
+}
diff --git a/tests/parser/tiny.cton.ref b/tests/parser/tiny.cton.ref
index 31f5767335..f2404bbf94 100644
--- a/tests/parser/tiny.cton.ref
+++ b/tests/parser/tiny.cton.ref
@@ -35,3 +35,9 @@ ebb0(vx0: f32, vx1: f32):
     v1 = fcmp uno, vx0, vx1
     v2 = fcmp lt, vx0, vx1
 }
+
+function bitcast(i32, f32) {
+ebb0(vx0: i32, vx1: f32):
+    v0 = bitcast.i8x4 vx0
+    v1 = bitcast.i32 vx1
+}